1. Field of the Invention
The present invention concerns a coupler that provides for a partial tapping of microwave energy propagated in a waveguide towards a microwave circuit. More precisely, it concerns a three-plate type of coupler comprising plungers that penetrate the guide.
2. Description of the Prior Art
In certain applications, it is necessary to tune the coupling between the guide and the microwave circuit very precisely at a pre-defined value. An application of this type is, for example, an electronic scanning antenna where there is a series of couplers supplying, from a common guide, a series of phase shifters which themselves supply the radiating elements. The radiation pattern desired for the antenna entails very precise coupling for each coupler. Since the coupling is given by the mechanical length of the plungers in the guide, the requirement of precision in the coupling implies a requirement of manufacturing precision for all the parts coming into play.
In practice, the manufacturing tolerances are such that the radiation pattern obtained is rarely satisfactory. Hence, after the antenna is mounted, tests and measurements are conducted to prepare corrected values for the length of the plungers. Then the couplers are dismantled, the plungers are replaced and then the couplers are re-mounted. The need for these various operations obviously entails disadvantages, especially as regards manufacturing time and costs, all the more so as the number of couplers is large. Furthermore, the operations for dismantling and re-mounting the coupler may cause the mechanical deformation of certain parts and may, consequently, introduce additional errors.
An object of the present invention is a coupler of this type which can be tuned without dismantling, thus avoiding the above disadvantages. To this end, the coupler has external means for the variation of the height of the central conductor of the three-plate structure at the level at which the plungers are fixed, thus causing a variation in the penetration of the plungers into the guide, and consequently, a variation in coupling.